Semiconductor laser diodes, manufactured as single emitter lasers or a laser diode bars, may have an electrical-to-optical conversion efficiency reaching 50% and higher, and can presently achieve optical power levels of a few Watts or even tens of Watts per a single emitter laser diode, and tens to hundreds of Watts per a laser diode bar. Due to high efficiency, reasonable power levels, and high spectral and directional brightness, laser diodes and laser diode bars find applications in many areas, such as material processing, offset printing, medical treatment, pumping of solid state lasers, and particularly pumping of fiber lasers.
There are two important considerations related to packaging of laser diodes into a single package. A first of these considerations is heat sinking. Laser diodes, in operation, generate considerable amounts of heat, since not all of the electrical energy used to power the laser diodes is converted into optical energy. The non-converted energy is released as heat. At an efficiency of 50% and optical power level of 5 W, for example, a single laser diode emitter will generate 5 W of heat. The heat needs to be removed so as to ensure stable and reliable laser diode operation. Moreover, since the central wavelength of laser radiation depends on laser chip temperature, the latter often needs to be stabilized with a typical accuracy of about one degree Celsius.
The second important consideration related to packaging is optical coupling. Due to a thin-slab geometry of laser diodes, their radiation, propagating along Z-axis, has a highly asymmetric lateral distribution of optical power density and divergence along X- and Y-axes. Assuming a standard notation of an X-axis lying in the plane of the laser diode slab, a Y-divergence of a laser diode is typically much higher than an X-divergence and is almost diffraction limited, whereas the X-divergence of a laser diode is usually smaller and is not diffraction limited. Such an asymmetry of laser diode beam poses a certain difficulty in applications where a symmetric, round beam is required, for example in applications involving coupling of radiation of many laser diodes into an end of a single optical fiber. Since an optical fiber generally has a substantially circular or polygonal cross-section and has a substantially symmetrical acceptance angle, the combined radiation of a diode laser has to be symmetric in its divergence and lateral power density distribution, in order to couple as much light into an optical fiber as possible.
A variety of ways of solving both abovementioned design considerations have been suggested in the prior art. For example in order to provide heat sinking, the laser diodes or laser diode bars of a diode laser apparatus are typically placed onto a common heat sink or stacked together. Even though stacking provides certain advantages, such as a simpler and more compact optical arrangement, it is not as efficient as a common heat sink. In a stacking arrangement, heat flows in a serial fashion, whereas when a common heat sink is provided, heat flows in a parallel fashion allowing more heat to be removed. On the other hand, a common heat sink method often results in bulky and inefficient coupling optics. For example, one prior art apparatus uses a complex multi-faceted reflector to combine beams from individual laser diode chips. The resulting device is expensive and difficult to align. Other prior art designs use waveguides or complex stair-like heat sink structures and microlenses combined with multi-faceted reflectors, which are utilized to combine the individual beams of laser diodes into a symmetrical output beam.
One important type of laser diode assembly is a single-bar assembly. In a single bar of laser diode emitters, the latter are formed on a common semiconductor substrate, side-by-side, and therefore allow for a parallel heat flow towards the common substrate to occur. Since the lateral position of individual emitters in a bar is precisely defined using photolithography, a simple and reliable pre-manufactured set of micro-optics can be used to collimate and reformat the output laser beam. In diode lasers made this way, a few tens of Watts of output power can be easily generated and coupled into an optical fiber. However, other problems such as warping, or so called smile of a bar, come into play reducing fiber coupling efficiency and device reliability; furthermore, it is not very easy to remove heat from individual emitters disposed with a sub-millimeter pitch on the common semiconductor substrate. There is also a reliability concern related specifically to single-bar diode lasers: when a single laser diode emitter fails catastrophically in a bar e.g. due to an electrical short, it often disables its neighboring emitters, failing the entire single-bar diode laser.
While there are many specific geometries presently available to package laser diodes into an assembly, it is clear that a successful device will naturally combine efficient heat removal from individual laser diode emitters with a simple, inexpensive, easy to align set of optics for reformatting highly asymmetrical anamorphic beams from individual emitters into a low aspect ratio, single optical beam suitable for optical fiber coupling, material processing, and a multitude of other tasks.
It is, therefore, an object of the present invention to provide an inexpensive and compact, fiber or free space coupled light source, for pumping fiber lasers and solid state lasers, material processing, medical treatment, offset printing applications, and the like.